As environmental legislation progresses, so the need grows for sensors with which even the smallest amounts of pollutants may be reliably determined. A major role is played in this context by, above all, measuring methods that enable the determination of gaseous pollutants in the ppm range. However, determination of the content of non-oxygen gases in particular, principally NOx (nitrogen oxides), in exhaust gases from combustion represents a particular challenge in this respect because of the oxygen fraction which is present at the same time. This is particularly important with regard to potential application as an on-board-diagnosis sensor (OBD sensor) for compliance with the coming exhaust gas legislation, since in that context limits will be laid down, for example for nitrogen oxides, that are below the resolution limit of currently available solid electrolyte gas sensors.
To measure small gas concentrations of non-oxygen exhaust gases, principally NOx, with an oxygen background present, use is made, in particular, of solid electrolyte sensors based on zirconium dioxide. In such sensors, there may be, for example, a plurality of sections or chambers which are separated from one another by diffusion barriers. Oxygen is removed at one or more first electrodes. Thus, ideally, oxygen is then no longer present in subsequent chambers, so that an electrode present in those chambers is then able to break down nitrogen oxides and pump the resulting oxygen as an ion current to a reference electrode or to another electrode. The very small electric current corresponding thereto is measured and is a measure, for example, of a nitrogen oxide concentration in the exhaust gas.
Sensor elements and methods for determining gas components in gas mixtures are described in German Patent Application Nos. DE 10 2008 040 314.8 and DE 10 2008 044 374.3. Described therein is a method for measuring a gas species in low concentration, in which accumulation of a quantity of oxygen equivalent to the concentration of the gas species to be measured is carried out by pumping into a gas-tight chamber with the aid of a pumping cell. A time interval until a characteristic concentration threshold of the accumulated gas is reached is determined. Then, a defined initial state is re-established in the gas-tight chamber by pumping.
A problem with the conventional methods and devices, however, is that the circuitry is generally very complex. In particular, for controlling the above-described method described in German Patent Application No. DE 10 2008 040 314.8 and DE 10 2008 044 374.3, to separate different functional cells of the sensor element it is necessary to use floating measuring devices for measuring voltage and current in order to avoid cross-talk between those functional cells.